Randal Olson analyzed the duration of "the 25 most popular movies from each year from 1931 through 2013," and reveals the truth behind the myth that movies are "getting so much longer than they used to be."
Up until the 1950′s, feature films grew by 15-30 minutes. Then after the 1950s, the average movie hovered around 90 minutes. Interestingly, the trend here shows that movies have been getting a little bit shorter in the past few years. We'll have to revisit this data in a few years to see if that trend holds.
The chart he created in January 2014 shows "the average feature film length over that time period."
Maybe they just seem longer because so many of them suck.